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Calmatives for the Excitable Horse: A Review of L-tryptophan

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(2000) using dogs, tryptophan reduced aggression, but did not reduce hyperactivity., at high doses tryptophan may reduce endur­ance capacity, which would have adverse consequences for horses involved in stamina-related pursuits such as racing, endurance and eventing., the effects of tryptophan on hyperactivity may be especially relevant to its use as a calmative in horses. how­ever, perhaps the most clinically significant aspect of bagshaw's study is that the level of l-tryptophan admin­istered was much lower than that contained in most commercial preparations, and no change was seen in blood concentrations of tryptophan or serotonin in the treated horses. however, behavioural characteristics more closely linked to excitement, such as hyperactivity in dogs, are not modified by tryptophan supplementation. theoretically, therefore, increased concentrations of tryptophan in the cns equate to increased serotonin synthesis (fern­strom and wurtman, 1972). mench and shea-moore (1995) ex­plained this apparent anomaly by suggesting that in dominant individuals, more tryptophan may be metabo­lised peripherally, and as such less is available for con­version to serotonin in the brain. the absence of direct scientific evi­dence that tryptophan does calm excitable horses, is also of concern. this condition is characterised by front-paw treading, hindlimb abduction, head weaving, and tail-lashing, and is reported to occur when low doses of tryptophan are given, possibly associated with a failure to saturate tryp­tophan hydroxylase. acute hemolytic anemia after oral administration of l-tryptophan in ponies. a second group treated once orally with a lower dose of tryptophan (350 mg/kg) showed less severe haemolysis, but also presented with increased respiratory rates with­in 24 h of dosing. although these obser­vations indicate an effect of tryptophan on mood or behaviour, they are of limited relevance to the treatment of excitable horses, and will not be explored further here (for reviews, see liebermann et al. interaction of dietary tryptophan on territorial aggression, motor activity, and neurochemistry in mice.

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first, some horses become more excitable when fed con­centrate diets, so the effects of a calmative would be more noticeable. tryptophan at high doses causes haemolysis in ponies, due to the production in the gut of a toxic metabolite (paradis et al. this delayed re­sponse is consistent with results obtained by laycock and ball (1990) in layer hens, where it took six days for tryptophan to alleviate hysteria. behavioural and growth effect of oral administration of rumen protected tryptophan on weanling beef calves. effect of tryptophan on the behaviour of nonstressed and stressed mice in porsolt's swim test. is a lack of direct evidence to support the use of tryptophan in horses, but the owner or veterinarian may be persuaded that tryptophan does have calmative prop­erties, from experience, or the weight of anecdotal evi­dence. thus, toxic effects on the blood and respiratory system occur in ponies fol­lowing oral tryptophan at 350 mg/kg (paradis et al. significantly, the response is species-dependent, and there are no scientific publications that confirm the efficacy of tryptophan as a calmative in excitable horses.­more, the study was conducted on a small number of animals (four horses per group), which is less than opti­mal for behavioural work. low doses of tryptophan may be excitatory in the horse, and no scientific studies have shown that tryptophan is an effective calmative in horses when used in the dose range provided by com­mercial preparations. it is hotly debated whether the epidemic was caused by l-tryptophan itself, or a con­taminant of the commercial product, but while the fda restrictions on human use remain in place today, l-tryptophan continues to be used in the us and else­where in the world, as a supplement for animals. the few tryptophan experiments that have been conducted in horses are examined in detail, and tryptophan research in other species is reviewed, in search of evidence to sup­port its use as a calmative agent. for evidence that tryptophan supplements may be beneficial in this regard, we are compelled to extrapolate from research conducted in other species.

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, the level of arousal could influence the re­sponse of horses to tryptophan, as it has been shown in cats that arousal is associated with a higher rate of fir­ing of serotonergic neurones (trulson and jacobs, 1979). the relative plasma concentration of lnaa can be increased by feeding a high protein diet that contains more lnaa than tryptophan, or may be decreased via the release of insulin, which appears to remove selectively the lnaa from plasma with less effect on tryptophan (clark and mills, 1997). the effect of dietary tryptophan on pig behaviour and meat quality - preliminary results. these preparations are sold as pastes or feed additive powders, with recommended tryptophan dosages ranging from 0. role of tryptophan as a cause of central fatigue is not universally accepted. discussed earlier, extrapolating the results of monogastric studies to horses could be dangerous be­cause of toxic hindgut metabolites. experiments have shown that tryptophan supplementation decreases aggression in humans, dogs, pigs, poultry, and fish, and that it may reduce fearfulness and stress in calves, vixens and poultry., when the mares were given tryptophan (100 mg/kg), their endurance was reduced significantly (' 16%), which is consistent with the effect seen when rats were treated with serotonin agonists (davies, 1995), and supports a role for tryptophan and serotonin in contributing to central fatigue. (1994) noted the possibility that there are breed differences in the basal level of serotonin activity in horses, whereas sex differences in hypothalamic sero­tonin concentrations have been confirmed in pigs, such that female swine have less serotonin and are more sen­sitive to changes in the tryptophan:lnaa ratio than males (henry et al. compared with horses fed mostly roughage, a greater calmative effect may be seen when tryptophan is given to horses on concentrate diets, for two reasons.), but caused no behavioural effects that were altered by tryptophan, regardless of the diet. tryptophan supplementation appears to be effective in reducing aggression and possibly fearfulness in some species, its impact on hyper-reactivity and stress are questionable. for example, newly-weaned calves spend more time lying down when supplemented with tryptophan, with less exploratory behaviour and vocalisation (nakanishi et al.
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equine calmative preparations are available commercially that contain l-tryptophan, mostly in com­bination with other `calming' ingredients such as thia­mine and magnesium., a third group of ponies given a single intravenous (iv) infusion of tryptophan (100 mg/kg) showed no clinical abnormalities, haemolysis, or haemoglobinuria (paradis et al. design considerations for the evaluation of tryptophan supplementation in the modification of equine behav­iour. the few studies where l-tryptophan has been administered to horses suggest that low doses (relative to those contained in commercial preparations) cause mild excitement, whereas high doses reduce endurance capacity and cause haemolytic anaemia [destruction of red blood cells] if given orally. summary, the type and level of dietary fat, protein, and carbohydrates, as well as the exercise status of a horse, could all influence tryptophan uptake into the cns, and the response to tryptophan administration.: to determine whether a commercial dose of l-tryptophan increases plasma tryptophan and alters behaviour in horses fed a roughage or concentrate meal. few studies on tryptophan have been conducted in horses, and none has produced direct evi­dence of calmative efficacy in this species. two to four hours after tryptophan dosing, isolated horses given 0. third and most recent paper on this topic focused on the central fatigue hypothesis, and the effects of tryp­tophan on exercise capacity in horses (farris et al. supplement my high strung gelding with b vitamins and a supplement that has l-tryptophan and i notice a big difference in his demeanor. perhaps it is basic measures such as these, which provide the most reliable, efficacious, and ethically acceptable solutions to problems associated with `excitability' in horses.” many calming supplements that contain l-tryptophan also contain other ingredients that are banned under usef rules, and in these instances the supplement would not be allowed. role of serotonin (and hence tryptophan) in cen­tral fatigue, is supported partly by evidence for a "ther­mal switch" in the hypothalamus, which limits exercise performance once the core temperature reaches a critical threshold (gandevia, 2001).

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however, the ratio of tryptophan to 4 large neutral amino acids (phenylalanine, tyrosine, leucine and isoleucine) increased in the tryptophan-treated horses to a similar extent and for a similar duration, with both diets. evidence for the role of tryptophan/serotonin in central fatigue, includes the observation that exercise increases tryptophan concentrations and serotonin turn­over in various parts of the cns (chaouloff et al. the acute effects of oral (-)-tryptophan in human subjects. lengel, dvm, says that under the usef’s drugs and medications rule, l-tryptophan is considered a nutrient, not a drug, and therefore is not a forbidden substance.: plasma tryptophan increases when tryptophan is administered at a dose used in some commercial products, but this is not reflected by marked behavioural changes in the horse. mg/kg appears to be too low, causing mild excitation in horses (bagshaw et al. suppression of aggression in rainbow trout (oncorhynchus mykiss) by dietary l-tryptophan. effort has been made to ensure the accuracy of the l-tryptophan powder-pure information published above. behavioural and physiological effect of orally administered tryptophan on horses subjected to acute isolation stress. furthermore, glucose infusions caused no change in plasma free fatty acid concentrations, or in the ratio of free tryptophan to lnaa, which is not con­sistent with the central fatigue hypothesis proposed by newsholme et al. because of competition between tryptophan and the lnaa for carrier binding sites, the rate at which tryptophan enters the brain is a function of both tryptophan and lnaa concentrations. in other circumstances, the needs of excitable horses may be served better by a variation in the exercise regime or hous­ing conditions (fraser, 1992). behaviour of horses is affected by soy lecithin and corn oil in the diet.

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behaviours induced by 5-hydroxytryptophan in neonatal, preweaning, post-weaning and adult sprague-dawley rats. effect of dietary protein content and tryptophan supplementation on dominance aggression, territorial aggression, and hyperactivity in dogs. this enzyme is only found in serotonin-forming cells, whereas the second reaction in the two-step conversion, the decarboxylation of 5-hydroxytryptophan, can occur in any mammalian cell (lovenberg et al. (2001) interpreted re­duced exploratory behaviour in tryptophan-treated mice, as increased fearfulness. depletion and the consequent lowering of brain serotonin have been linked to increased aggres­sion in humans (young, 1991), whereas tryptophan sup­plements have been used with some success in treating pathologically aggressive people (young, 1991). an in­crease in free fatty acids following a high fat meal for example, can result in elevated free plasma tryptophan, and ultimately, more serotonin production (newsholme et al. g) or placebo (water) was administered per os in a cross-over design, to 12 thoroughbred horses (503 +/- 12. a diet that contains more fat might have a calmative effect itself, which could either enhance or mask any effect of tryptophan supplementation. cautionary note regarding the use of nutritional l-tryptophan to alter aversion-related behaviour in mice. for performing study: l-tryptophan is a common ingredient in equine calmative products, but its effectiveness has not been demonstrated in horses. mg/kg), and the horses were isolated in an enclosed stall. thus, in male chickens, tryptophan supplementation decreases aggressive pecking, especially in the more dominant birds (shea et al. tryptophan is con­verted to serotonin via a two-step reaction.

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is difficult to recommend a therapeutic, or even a safe dose rate for using l-tryptophan in horses as a calmative. (1988) characterised nervous or highly-reactive horses as those with a high activity index (horses that spend more time walking, trotting, cantering and galloping, than `normal' horses), fraser (1992) described hyper-reactivity (or hyperactiv­ity) in terms of aggression (mobile aggression, mobile alarm, threatening, biting, kicking) and aversion. in australia* are warning that there “is an urgent need for research” to confirm the efficacy and safety of l-tryptophan supplementation in horses. as both conditions favour more tryptophan entering the cns, increased production of brain seroto­nin might be expected, and this could account for the decreased motor drive and increased sensation of fatigue, experienced after a period of strenuous exercise (news­holme et al. as tryptophan continues to be used as an equine calmative, there is an urgent need for research to confirm its efficacy in horses, and to establish a safe therapeutic dose range. experiments designed to evaluate tryptophan as a calm­ative have not always considered these factors. if tryptophan is to be used, it is well to consider the many factors that could influence the effi­cacy of a supplement. accordingly, great care must be taken when extrapolating to horses, the results of tryptophan studies conducted in humans, or other animal species. the ban was issued after a link was established between l-tryptophan supplementation and eosinophilia-myalgia syndrome, a potentially deadly disease in humans that can lead to congestive heart failure., 1968), or after oral dosing with rumen-protected tryptophan (nakanishi et al. nevertheless, considering species differences and the range of factors that may influence the response to tryptophan, it would be impru­dent to guarantee a tryptophan calmative response in any individual. relevant to horses, is evidence that tryptophan may affect fearfulness. laycock and ball (1990) fed a diet containing 5 g tryptophan/kg feed to a commercial layer flock of 10,000 hens suffering from hysteria, and showed that the symptoms were abolished two days after the cessation of a six-day course of treat­ment.

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serum tryptophan level after carbohydrate ingestion: selective decline in non albumin-bound tryptophan coincident with reduction in serum free fatty acids. although the iv dose was the lowest used, plasma tryptophan concentrations were elevated to the same extent as seen following oral treatment.., competitions) may reduce the incidence and severity of excitability in horses., the rationale for tryptophan use in horses is that increasing the plasma concentration of tryptophan will lead to increased tryptophan uptake by the cns, and increased serotonin production in the brain. however, he says, “l-tryptophan, when administered for the purpose of calming a horse in competition, violates the spirit of the usef’s rule. (1991a), no signs of sedation were reported in ponies given the tryptophan infusion, while those treated orally became restless. a similar effect is seen in cattle, where rumen microorganisms con­vert tryptophan to 3-methylindole, which causes severe acute respiratory disease (fog fever'). newberry and blair (1993), who observed that tryptophan-supplemented chickens flap less when handled, reached a similar con­clusion. this im­plies an association between serotonin and behaviour in horses, and suggests that there may be breed differ­ences in the behavioural response to tryptophan administration. moreover, tryptophan infusion has been shown to reduce endurance in horses (farris et al. endurance times were recorded in seven mares exercised to fatigue on a high-speed treadmill, and trea­ted with tryptophan and (or) glucose infusions in a cross-over design. are not aware of any studies on the long-term use of supplemental dietary tryptophan in horses, however it seems not to be harmful at 2 g/day in humans (leath-wood, 1987), or at 5 g/kg feed in chickens (laycock and ball, 1990). this paper outlines the proposed mechanism of action of tryptophan, and dis­cusses nutritional factors that influence tryptophan up­take into the central nervous system.

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one us manufacturer claims that "there are no known side-effects in horses from the supplementation of tryptophan" (richdel inc. behavioural responses of broiler chickens to handling: effects of dietary tryptophan and 2 lighting regimens. with tryptophan, a horse makes niacin, a b vitamin, which is important for normal digestion, skin, nerves, and serotonin. that contain tryptophan are marketed world wide as calmative agents to treat excitable horses. a naturally occurring amino acid found in human foods such as turkey, chicken, pork and cheese, l-tryptophan is also a main ingredient in several equine calming supplements on the market today. (1999), and indicates that gender could be a critical factor in the response of horses. my horse has colicked twice now, both times the day after i gave him "so calm" which contains l-tryptophan. presumably, the supplements limited tryptophan entry in to the cns, and reduced serotonin synthesis. hysteria gradually returned after the tryptophan supplementation ceased, although it was eliminated without return when in a subsequent experiment, trypto­phan was administered to the entire barn, rather than just one room. secondly, carbohydrate diets with a high glycaemic index cause more insulin release, more uptake of lnaa into peripheral tissues, and less com­petition with tryptophan for uptake into the brain (fig. while this brings into question the efficacy of trypto­phan as a calmative in horses, it should be noted that the doses used were between 27 and 750 times higher than those found in commercial preparations. oil in horse feed feeding for hoof health and barefoot trimming feeds and feeding for glossy coat, mane and tail feeds that provide high digestible fibre and low nsc for horses feeding your horse for better health and performance non structural carbohydrate sugar and starch (nsc) in horse feed. thus, lyons and truswell (1988) showed a positive associa­tion in humans, between diets with a high glycaemic in­dex, insulin concentrations, and the ratio of tryptophan to lnaa in plasma.

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the few studies where tryptophan has been administered to horses suggest that low doses (relative to those contained in commercial preparations) cause mild excitement, whereas high doses reduce endurance capacity, and cause acute haemolytic anaemia if given orally, due to a toxic hindgut metabolite. reduced territorial behaviour and aggressive dominance is also seen in dogs given tryptophan (denapoli et al. however, two other issues should be consid­ered when contemplating the suitability of tryptophan for this application. is a very interesting article to me as i use a calming supplement when i show my gelding and they almost always contain l-tryptophan., 1973), but also lim­its the competition between tryptophan and the lnaa for transport into the cns. they also ar­gued that the use of mature mares was inappropriate because the blood-brain barrier becomes less permeable to tryptophan with age (although no evidence was given to support this assertion in horses). (1994) point out, studies in other species have also failed to de­tect increased blood levels, despite increased brain con­centrations of tryptophan and serotonin after oral dosing (fernstrom and wurtman, 1972, 1974). feeding and nutrition of horses: the making of a champion. national ban ended in 2001, with sales of tryptophan commencing again in 2002. (1994) concluded that at the level administered, tryptophan had no sedative effect, but seemed to cause a mild excitation (bagshaw et al. this was due to the association between the use of l-tryptophan supplements made by a japanese company, and a us epidemic of eosinophilia ­myalgia-syndrome (ems), involving more than 1500 reported cases, and at least 38 deaths (for review see hertzman et al. respiratory symp­toms are not observed in cattle after iv infusion of tryptophan (carlson et al.-tryptophan is an essential amino acid precursor to serotonin, a critical neurotransmitter for stabilizing mood, relieving panic and anxiety, and maintaining a normal sleep-wake pattern.
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it is important to understand these factors to be able to evaluate critically the results of tryptophan experiments. tryptophan in the form of l-tryptophan is considered 100% digestible and available to the animal. it is not known how much oral tryptophan reaches the hind gut in horses and undergoes microbial metabolism, but it is known that the conversion of tryptophan to indole (but not 3-methyl indole) occurs in caecal fluid in vitro (paradis et al. its popular use as a `natural' calmative for horses, few scientific studies of tryptophan have been conducted in this species, and only one has focused on behavioural effects. training meth­ods and more thorough acclimatisation of horses to potentially stressful situations and environments (e. the first and rate-limiting step is the conversion of l-tryptophan to 5-hydroxytryptophan, catalysed by tryptophan hydroxylase (boadle-biber, 1993). it’s long believed that l-tryptophan has a calming effect in animals, including humans and horses. how­ever, even without tryptophan administration, several nutritional factors can influence the passage of trypto­phan across the blood-brain barrier, and alter serotonin production (fig. relevance: further work is required to refine behavioural tests and identify an effective dose of l-tryptophan in the horse. acidosis in horses anhdrosis or puffs choke colic dod or splints equine metabolic syndrome fizzy, hot and excitable behaviour gastric ulcers greasy heel or mud fever insulin sensitivity tying up and pssm leaky gut sand colic urticaria or hives.™ l-tryptophan powder may be given for stress from traveling, hospitalization, moving, or other situations that may cause anxiousness and may also support a normal disposition. concentrations of tryptophan, serotonin and associated metabolites, were also found in the brains of tryptophan-supplemented birds (laycock and ball, 1990). product contains l-tryptophan which may be prohibited in certain competitions..

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this could account for the sex difference in the response to tryptophan noted in the fox by rouvinen et al. pigs subjected to pre-slaughter stress perform fewer aggressive acts if sup­plemented with 5 g tryptophan/kg feed (~55 mg/kg body weight) for five days pre-slaughter, but with no reduction in the incidence of pse meat (warner et al. furthermore, glucose infusions caused no improvement in endurance when tryptophan was given. has to when the author last researched the information about tryptophan. growth performance and brain neurotransmitters in pigs as affected by tryptophan, protein, and sex. per­haps a closer consensus on what constitutes excitability (fearfulness versus mobility or aggression) is needed, be­fore the effects of tryptophan can be properly assessed. (1998) showed that by causing central fatigue, tryptophan can reduce endur­ance, regardless of substrate supply. it is known, however, that the effects of tryptophan on behaviour are species dependant, as well as being subject to the influence of diet, exercise, age, gender, breed, social sta­tus, and level of arousal. (1996) found that activity and reactivity were reduced in horses consuming a diet containing 10% corn oil. (1994) administered l-tryptophan to 10 mature arabian and standardbred mares subjected to isolation stress.”in 1989 the food and drug administration issued a nationwide recall, which is still in place today, of all over-the-counter dietary supplements for humans containing 100 milligrams or more of l-tryptophan, and in 1990 it prohibited importation of the substance. research using a variety of animals other than horses, has shown that the behavioural response to tryptophan supplementation varies with age, breed and gender, and can be modified by diet, exercise, social status, and level of arousal. on the available evidence, it would be incau­tious to rely on tryptophan supplementation to calm the excitable horse, especially when other measures might be available to address the underlying cause of excitability, such as altering diets, management and hus­bandry procedures.

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in humans, for example, a fourfold in­crease in the ratio of plasma tryptophan to lnaa, re­sults in double the concentration of tryptophan in the brain, and a 20% increase in brain serotonin (leath-wood, 1987). this binding is believed to restrict the tissue uptake of tryptophan in response to insulin (lipsett et al. interactive effects of dietary levels of tryptophan and protein on voluntary feed intake and growth performance in pigs, in relation to plasma free amino acids and hypothalamic serotonin. (1998), there was a tendency for the mares to use less muscle glycogen during exercise, if they were given tryptophan. accordingly, young (1991) has predicted that the effects of tryptophan would be greatest when ani­mals are aroused, as would be expected in the case of the excitable or stressed horse. after the peak, tryptophan remained high for several hours if the horses had been fed hay, but fell sharply if fed oats, consistent with the glycaemic responses to these meals. in the meantime, available data suggest that it would be impru­dent to rely on tryptophan to calm the excitable horse, and instead, that a greater effort should be made to identify the underlying causes of excitability, and to explore more appropriate non-pharmacological remedies. short duration of bagshaw's study should also be considered in view of findings in other species, that behavioural changes are not observed less than a week after commencing daily tryptophan supplementation (laycock and ball, 1990; winberg et al. caution must be taken when used on competition horses subject to drug testing. thus, better dietary management could be more appropriate than tryptophan supplementation in many cases. and mills (1997) have cautioned that age may decrease the response to tryptophan, because of de­creased permeability of the blood-brain barrier. finally, the concentration of tryptophan in the protein component of a horse's diet, relative to the concentration of other large neutral amino acids, will determine whether supplementing with additional tryptophan is likely to provide any benefit. however, the warning out of australia cites that l-tryptophan’s calming effect is “species-dependent, and there are no scientific publications that confirm the efficacy of l-tryptophan as a calmative in excitable horses.
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the interaction of dominance status and supplemental tryptophan on aggression in gallus domesticus males. estimates of the level of tryptophan binding vary from 20% to 90%, depending in part on the concentration of free fatty acids, which compete with tryptophan for binding to albumin. (1999) re­port that tryptophan also tends to reduce exploratory behaviour in male silver foxes, whereas the opposite ef­fect is seen in female foxes. *“calmatives for the excitable horse: a review of l-tryptophan. a group of adult shetland ponies given a single dose of tryptophan (600 mg/kg) by stomach tube, exhibited signs of restlessness, elevated respiratory rate, haemolysis, and haemoglobinuria. furthermore, although tryptophan was reported to potenti­ate territorial aggression in male mice (lasley and thurmond, 1985), the majority of studies report de­creased aggression in a variety of animal species. based on a review by leathwood (1987) of human clinical studies, tryptophan is an effective seda­tive at 500 mg (-7 mg/kg) if given with a carbohydrate load. in other human studies, 10 g of tryptophan has been ingested (-140 mg/kg) without reported ad­verse effects (bellisle et al. the lack of information which establishes a safe and effective oral dose for l-tryptophan in horses, is of con­siderable concern. depending on the manufacturer, claims of calmative efficacy are either express or implied, with at least one australia-based manufacturer claiming that tryptophan can also enhance athletic performance (nature vet).. 2005, review: calmatives for the excitable horse: a review of l-tryptophan. most amino acids, tryptophan circulates in the plasma bound extensively to plasma proteins, chiefly albumin (mcmenamy and oncley, 1958). under normal condi­tions in mammals, tryptophan hydroxylase is not fully saturated by its substrate, and no negative feedback mechanism from serotonin is known.